1. Field of the Invention
The present invention relates to a transmission system having a transmitter comprising coding means for deriving from a plurality of sequences of digital symbols a plurality of sequences of coded digital symbols, modulation means for deriving from said plurality of sequences of coded digital symbols a plurality of digitally modulated carrier signals, and combining means for deriving a combined signal from said plurality of digitally modulated carrier signals. The transmitter is arranged for transmitting said combined signal over a non linear transmission medium to a receiver. The receiver comprises demodulation means for deriving at least one sequence of coded symbols from the received signal, and decoding means for deriving a sequence of digital symbols from said sequence of digital symbols.
The present invention also relates to a transmitter, a coder/demodulator, a receiver and a signal for use in such a system.
2. Description of the Related Art
A transmission system according to the preamble is known from "Coding to increase number of channels in QAM-SCM-IM/DD lightwave system" by J. H. Wu, Y. H. Lee and J. Wu in Electronic letters, 2.sup.nd January 1992, Vol. 28, No. 1, pp.65 . . . 67.
Such transmission systems can find their application in all kinds of transmission networks having a non linear transmission medium.
A first example of such a transmission system is an optical transmission system. The transmission system is here regarded to comprise an electro-optical converter, e.g., a laser, an optical transmission path such as an optical fibre or free space, and an opto electrical converter such as a photo diode. In common optical transmission systems the non-linearity of the system is mainly caused by the electro-optical converter, but it is also possible that substantial non-linearity is caused by optical amplifiers present in the transmission path.
A second example of a transmission system according to the preamble is a cable transmission network such as a CATV network. Here the non-linearity is mainly caused by the non linearity of the amplifiers used at several places in the network.
In transmission networks it is often desired to transmit a plurality of sequences of digital symbols. Each of said sequences of digital symbols represents, e.g., a separate (HD)TV program or a data service. In the known transmission system each such sequence is coded to obtain a sequence of coded digital symbols. Subsequently the sequence of coded digital symbols is modulated on a proper carrier to obtain a modulated carrier signal. Finally, the different carrier signals are combined to form a combined signal which is transmitted via the transmission medium. In the receiver at least one of the modulated carrier signals is selected, demodulated and decoded to obtain the desired sequence of digital symbols.
Due to the non-linearity of the transmission medium the combined signal will undergo some non-linear distortion, resulting in the generation of harmonics and intermodulation products. The presence of such distortion products will cause some interference to the desired signals leading to an increased error probability.
In the known transmission system the sequences of digital symbols are coded using a (255,239) BCH error correcting code to reduce the error probability of the received digital symbols. Due to the use of such an error correcting code, more distortion can be tolerated, and consequently larger signal amplitudes of the combined signal can be tolerated. This leads to an increased capacity of the transmission system. However the desire to increase the capacity of the transmission system even more still exists.